IGFs-I and -II are single chain polypeptides (70 ad 67 aa respectively) structurally similar to proinsulin, synthesized ubiquitously and circulating carried by specific binding proteins. The IGFs act through two types of specific membrane receptors: the so called "type 1" or IGF-I receptor, and the "type 2" or IGF-II receptor. In physiological conditions IGF-I is correlated with GH secretion. However it is also influenced by other hormonal and non-hormonal factors. Little is known about the physiological significance of IGF-II, whose concentration, at variance with IGF-I, is not increased in acromegalic subjects. It has been recently postulated that IGF-II has a critical role in prenatal growth, while IGF-I exerts its action after birth. IGF-I is important in the clinical evaluation of growth defects and of acromegalic patients. As to growth defects an overlapping of values is reported between normal and hypopituitary subjects under 5-6 years of life. However, although it is possible that a normal subject presents with a reduced IGF-I concentration, it never happens to find a hypopituitary subject with normal values. It is, therefore, possible to establish a cut-off value (100 micrograms/l in our experience) over which the occurrence of hypopituitarism is excluded. On the contrary IGF-I concentration is extremely useful in the evaluation of acromegaly, in particular to evidence the disease activity status. Moreover the IGF-I concentration seems to be useful to evaluate the nutritional status, particularly in association with serum transferrin, in subclinical conditions and in the monitoring of the nutritional therapy.